| Titanium and its alloys are popular structural and functional materials due to their excellent properties,but their wider application is limited by high price.NNF technology has attracted much attention because of its high utilization rate of materials,short process and the potential of greatly reducing the production cost of titanium products.Titanium and its alloy powders are usually the raw materials for NNSF,which are mostly produced by hydrogenation-dehydrogenation(HDH)and atomization methods currently.Since the raw materials for the two methods are high-quality titanium sponge,titanium alloy ingot,bar,wire,etc.,the high cost of the raw materials leads to the high cost of the powders,which remarkably weakens the advantages of NNSF.Thus,it is in great demand to develop a new technology to produce high-quality titanium powder with lost cost.Based on the breakthrough of hydrogen assisted Mg deep deoxidation process,a new solution to prepare titanium alloy powder by blended elemental sintering with high densification and deep deoxidation is proposed in this thesis.The requirement on the oxygen content of the raw materials in this method is greatly reduced,the selection of the raw materials becomes more flexible,and the traditional process of high-temperature smelting is replaced by sintering.It is expected to realize the low-cost preparation of titanium alloy powder.This thesis aims at the preparation of Ti-6Al-4V alloy powder,focuses on the sintering rules of alloying,densification,and homogenization when replacing with low-valued raw materials of titanium,aluminum,and vanadium,and optimizes the sintering process.The whole process of the new method of preparing Ti-6Al-4V alloy powder has been achieved.The main conclusions are as follows:(1)The self-made raw materials of crude titanium and crude vanadium obtained by reducing oxides with Mg have an irregular appearance and pore structure.The oxygen content in crude titanium is 2.03 wt%,and the hydrogen content is 3.31 wt%.The oxygen content in crude vanadium is 0.8255 wt%,and the hydrogen content is 1.21 wt%.It shows morphology inheritance of the crude titanium and crude vanadium with the oxide raw materials.(2)Using the radial shrinkage as the sintering effect,it is found that the sintering shrinkage rate is related to the raw materials of the sintering precursor.When mixing and sintering crude titanium with VAl alloy,the sintering shrinkage is the largest and the sintering densification is the densest.It is also demonstrated that the high specific surface area of the self-made crude titanium with the porous structure improves the sintering activity.The porosity by sintering crude titanium powder with VAI alloy powder at 1200? is 0.48%,which is much lower than that of 3.02%by sintering low-oxygen titanium powder with VAl alloy powder under the same sintering conditions.(3)The oxygen contents of the sintering precursors obtained by the various formula of mixing raw materials are significantly different,and the sintering rules and microstructures are also different.It is found that the main peak position of the a phase shifts to a low diffraction angle with the increased oxygen content of the matrix,the ?phase gradually weakens until disappears,and even the Ti6O peak appears.Last but not least,the typical TC4 basket structure also disappears with the increase of oxygen content.Generally,the sintering conditions to ensure the sintering effect are higher and higher as the oxygen content of the precursor increases.For example,the minimum condition required for the sintering of low-oxygen titanium and VAl alloy is 1000?/8h,but when sintering crude titanium with VAl alloy,the minimum condition is 1100?/2h;further,the minimum condition for sintering crude titanium,crude vanadium,and aluminum after mixing and pressing is increased to 1200?/2h.In a word,the alloying effect can be enhanced by improving the sintering conditions.(4)The densification is greatly reduced when replacing the VAl alloy with a single element,and it is difficult to improve the densification by strengthening the sintering conditions,which is closely related to the diffusion difference among titanium,aluminum,and vanadium.When titanium is sintering with VAl alloy,the diffusion rate of Ti and Al is faster than V,the Ti3Al phase is formed in the initial stage of sintering,and V cores can be observed,which will not lead to the residual pores caused by the fast diffusion of Al.When VAl alloy powder is replaced by Al powder and crude V powder,Ti-Al and Ti-V binary diffusion dominate in the initial stage of sintering,and Ti3Al phase and residual pores are formed due to the rapid diffusion of Al into the titanium matrix.The introduction of a single Al source is the key reason for the reduction of sintering densification.(5)An optimized sintering process of alloying aluminum source with other materials first and then sintering at higher temperature is proposed and can improve the densification,based on the reason for residual pore formation.V and Al pre-alloyed powder can be prepared by mixing the aluminum powder with crude vanadium and sintering them at 800? or 900?.The Al phase disappears,the distribution of Al and V is uniform,and the morphology of the pre-alloyed powder is similar to that of the crude vanadium.Sintering the self-made Ti with the pre-alloyed powder at 1100?,the cross-section porosity of the sintered product is reduced to about 0.3%.The combination of the two-step sintering of low-temperature pre-alloying and high-temperature sintering can achieve high densification and alloying with high oxygen in the matrix when titanium sponge and VAl alloy are completely substituted.(6)An optimized process of preparing Ti-6Al-4V alloy powder with low cost has been demonstrated.Hundred gram-scale Ti-6Al-4V alloy powder has prepared,which is highly densified with a density of 4.4285 g/cm3,a mean particle size of 71.6 ?m,and an oxygen content of 0.353 wt%.Compared with the standard composition of the TC4 alloy,the main element contents of Ti,Al,and V meet the standard,and the impurities of Si and O are slightly out of the standard but controllable. |
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